Follow-up from "Publish the feature - Limit pipeline job concurrency using named semaphores"

The following discussion from !21617 (merged) should be addressed:

• @ashmckenzie started a discussion:

  When the `resource_group` key is defined for a job in `.gitlab-ci.yml`,
  job executions are mutually exclusive across different pipelines for the same project.

  WDYT?

• @ashmckenzie started a discussion:

  only one of the jobs will be picked by the Runner, and the other jobs will wait until the

  WDYT?

cc @marcel.amirault, @dosuken123